Chapter 17 Reactions of Aromatic Compounds

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Chapter 17Reactions of Aromatic Compounds
Organic Chemistry, 5th EditionL. G. Wade, Jr.
Jo Blackburn Richland College, Dallas, TX Dallas
County Community College District ã 2003,
Prentice Hall
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Part 1 - Electrophilic Aromatic Substitution

• Electrophile substitutes for a hydrogen on the
  benzene ring.

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Mechanism
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Energy Diagramfor aromatic electrophilic addition
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Reactions

• Nitration HNO3 and H2SO4
• Bromination Br2 and FeBr3
• Chlorination Cl2 and AlCl3
• Iodination I2 and HNO3
• Sulfonation SO3 (fuming sulfuric acid)
• Alkylation R-Cl and AlCl3
• Acylation RCOCl and AlCl3

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Nitration of Benzene

• Use sulfuric acid with nitric acid to form the
  nitronium ion electrophile.

NO2 then forms a sigma complex with benzene
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Nitration of Toluene

• Toluene reacts 25 times faster than benzene. The
  methyl group is an activator.
• The product mix contains mostly ortho and para
  substituted molecules so methyl group is ortho,
  para directing.

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Sigma Complex

• Intermediate is more stable if nitration occurs
  at the ortho or para position.

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Energy Diagram
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Activating, O-, P-Directing Substituents

• Alkyl groups stabilize the sigma complex by
  induction, donating electron density through the
  sigma bond.
• Other kinds of groups with a lone pair of
  electrons stabilize the sigma complex by
  resonance.

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The Amino Group

• Aniline reacts with bromine water (without a
  catalyst) to yield the tribromide. Sodium
  bicarbonate is added to neutralize the HBr thats
  also formed.

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Summary ofActivators
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Ortho Substitutionon Nitrobenzene
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Para Substitution on Nitrobenzene
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Meta Substitutionon Nitrobenzene
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Deactivating Meta-Directing Substituents

• Electrophilic substitution reactions for
  nitrobenzene are 100,000 times slower than for
  benzene.
• The product mix contains mostly the meta isomer,
  only small amounts of the ortho and para isomers.
• Meta-directors deactivate all positions on the
  ring, but the meta position is less deactivated.
  

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Energy Diagram
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Structure of Meta-Directing Deactivators

• The atom attached to the aromatic ring will have
  a partial positive charge.
• Electron density is withdrawn inductively along
  the sigma bond, so the ring is less electron-rich
  than benzene.

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Summary of Deactivators
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More Deactivators
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Summary of Directing Effects
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Multiple Substituents

• The most strongly activating substituent will
  determine the position of the next substitution.
  May have mixtures.

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Bromination of Benzene

• Requires a stronger electrophile than Br2.
• Use a strong Lewis acid catalyst, FeBr3.

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Chlorination and Iodination

• Chlorination uses AlCl3 as the Lewis acid
  catalyst.
• Iodination requires an acidic oxidizing agent,
  like nitric acid, which oxidizes the iodine to an
  iodonium ion.

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Halobenzenes

• Halogens are deactivating toward electrophilic
  substitution, but are ortho, para-directing!
• Since halogens are very electronegative, they
  withdraw electron density from the ring
  inductively along the sigma bond.
• But halogens have lone pairs of electrons that
  can stabilize the sigma complex by resonance.
  

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Sigma Complexfor Bromobenzene
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Energy Diagram
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Sulfonation

• Sulfur trioxide, SO3, in fuming sulfuric acid is
  the electrophile.

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Desulfonation

• All steps are reversible, so sulfonic acid group
  can be removed by heating in dilute sulfuric
  acid.
• This process is used to place deuterium in place
  of hydrogen on benzene ring.

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Friedel-Crafts Alkylation
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Formation of Alkyl Benzene
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Limitations ofFriedel-Crafts

• Reaction fails if benzene has a substituent that
  is more deactivating than halogen.
• Carbocations rearrange. Reaction of benzene with
  n-propyl chloride and AlCl3 produces
  isopropylbenzene.
• The alkylbenzene product is more reactive than
  benzene, so polyalkylation occurs.

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Friedel-Crafts Acylation
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Gatterman-KochFormylation

• Formyl chloride is unstable. Use a high pressure
  mixture of CO, HCl, and catalyst.
• Product is benzaldehyde.

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Clemmensen Reduction

• Acylbenzenes can be converted to alkylbenzenes by
  treatment with aqueous HCl and amalgamated zinc.

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Part 2- NucleophilicAromatic Substitution

• A nucleophile replaces a leaving group on the
  aromatic ring.
• Electron-withdrawing substituents activate the
  ring for nucleophilic substitution.

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Examples ofNucleophilic Substitution
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Addition-EliminationMechanism
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Benzyne Mechanism

• Reactant is halobenzene with no
  electron-withdrawing groups on the ring.
• Use a very strong base like NaNH2.
  
  

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Benzyne Intermediate
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Part 3- Addition ReactionsChlorination of Benzene

• Addition to the benzene ring may occur with high
  heat and pressure (or light).
• The first Cl2 addition is difficult, but the next
  2 moles add rapidly.
• The product, benzene hexachloride, is an
  insecticide.

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Catalytic Hydrogenation

• Elevated heat and pressure is required.
• Possible catalysts Pt, Pd, Ni, Ru, Rh.
• Reduction cannot be stopped at an intermediate
  stage.

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Part 4 Side Chain ReactionsBirch Reduction
Regiospecific

• A carbon with an e--withdrawing group
• is reduced.

• A carbon with an e--releasing group
• is not reduced.

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Birch Mechanism
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Side-Chain Oxidation

• Alkylbenzenes are oxidized to benzoic acid by hot
  KMnO4 or Na2Cr2O7/H2SO4.

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Side-Chain Halogenation

• Benzylic position is the most reactive.
• Chlorination is not as selective as bromination,
  results in mixtures.
• Br2 reacts only at the benzylic position.

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SN1 Reactions

• Benzylic carbocations are resonance-stabilized,
  easily formed.
• Benzyl halides undergo SN1 reactions.

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SN2 Reactions

• Benzylic halides are 100 times more reactive than
  primary halides via SN2.
• Transition state is stabilized by ring.

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Part 5 - Reactions of Phenols

• Some reactions like aliphatic alcohols
• phenol carboxylic acid ? ester
• phenol aq. NaOH ? phenoxide ion
• Oxidation to quinones 1,4-diketones.

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Quinones

• Hydroquinone is used as a developer for film. It
  reacts with light-sensitized AgBr grains,
  converting it to black Ag.
• Coenzyme Q is an oxidizing agent found in the
  mitochondria of cells.

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ElectrophilicSubstitution of Phenols

• Phenols and phenoxides are highly reactive.
• Only a weak catalyst (HF) required for
  Friedel-Crafts reaction.
• Tribromination occurs without catalyst.
• Even reacts with CO2.

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End of Chapter 17